A recent Sports Medicine – Open study conducted a randomized controlled trial to judge how various carbohydrate intake and the glycemic index (GI) impact performance in endurance training regimens in men.

Study: Effects of a 10-Week Exercise and Dietary Intervention with Variable Dietary Carbohydrates and Glycaemic Indices on Substrate Metabolism, Glycogen Storage, and Endurance Performance in Men: A Randomized Controlled Trial. Image Credit: Lucky Business / Shutterstock.com

Background

Carbohydrates are crucial in maintaining performance during long sessions of endurance exercise. Optimal metabolic flexibility combined with full glycogen stores are essential prerequisites for prime endurance performance.

Energy requirements within the muscles increase dramatically while transitioning from rest to exercise. At the moment, fats or carbohydrates are used to provide adenosine triphosphate (ATP). With the growing intensity of exercise, carbohydrates turn out to be the important thing energy source, thus replacing fat.

An extended-term food plan low in carbohydrates and high in fat (LCHF) increases maximal fat oxidation at rest and through submaximal exercise conditions. Previously, the LCHF food plan has been shown to enhance the respiratory exchange ratio (RER) but not the time to exhaustion (TTE), even when glycogen stores were replenished shortly before the beginning of the competition.

Due to this fact, performance at high intensity is restricted as a consequence of reduced glycogen stores and mitigated carbohydrate metabolism. Thus, the LCHF food plan may impact physical performance by reducing training capability, exercise economy, and well-being when exercising at higher intensities, along with increasing the danger of fatigue, poor concentration, and gastrointestinal (GI) effects.

A high-carbohydrate food plan with low GI has the potential to beat the constraints often related to the LCHF food plan. In reality, recent studies have reported that any such food plan can result in improved metabolic flexibility and, because of this, superior performance improvements during an incremental cycling test. Nonetheless, few long-term studies have been conducted that assess the impact of a low GI food plan relative to an LCHF food plan on performance outcomes and metabolic flexibility.

In regards to the study

The present 10-week interventional study aimed to evaluate and compare the results of the LCHF food plan, a carbohydrate-rich high-GI food plan (HIGH-GI), and a carbohydrate-rich low-GI food plan (LOW-GI) on metabolic parameters, running economy (RE), peak running speed (PRS), and peak oxygen consumption. These parameters were assessed by measuring muscle energy stores, a five-kilometer time trial (TT) performance, and a graded exercise test.

The testable hypothesis was that the LOW-GI food plan can influence fat oxidation without restricting carbohydrate oxidation to the same extent because the LCHF food plan. Moreover, the LOW-GI and HIGH-GI groups were hypothesized to experience similar improvements in TT and PRS. Muscle glycogen stores were expected to say no within the LCHF food plan, whereas no significant difference was anticipated within the HIGH-GI and LOW-GI groups.

Study findings

The LOW-GI food plan led to reduced energy intake, decreased blood lactate concentrations during exercise, higher values within the graded exercise test, maintenance of glycogen stores, and improved TT performance.

The LCHF food plan also enhanced fat oxidation within the incremental test. Nonetheless, as a consequence of insufficient carbohydrate provisions, the LCHF food plan altered carbohydrate oxidation, muscle glycogen restoration, and training adaptions at higher intensities.

HIGH-GI improved performance at high intensities and increased muscle glycogen content. Nonetheless, after 10 weeks, fat oxidation was impaired.

Importantly, the LCHF food plan could negatively affect long-term health status despite its promising effects on fat oxidation and body composition. The lower intake of essential macronutrients and high-fat content could contribute to those antagonistic effects; subsequently, any such food plan must be really helpful with appropriate caution.

Plasma lactate concentrations were reduced within the LOW-GI group, whereas carbohydrate metabolism was unaltered during higher intensities. These two aspects, coupled with the facilitation of fat utilization, resulted in improved metabolic flexibility.

Within the HIGH-GI group, muscle glycogen levels increased significantly. Nonetheless, while responding to different exercise intensities, the changes in metabolism could impair the flexibility to change from carbohydrate to fat oxidation.

Overall, the study findings provide evidence that relative to the LCHF or HIGH-GI food plan, the LOW-GI food plan could lead on to useful changes in substrate oxidation during prolonged periods of exercise and improve endurance performance.

Study limitations

Weight loss plan monitoring was conducted by self-reported 24-hour recalls, which could have led to recall, reporting, and training biases. Nonetheless, these calls were accomplished twice per week, which should reduce the probability of random errors. Moreover, an extra food frequency questionnaire was used to reduce errors and assess food plan status before the study.

No differences across diets were noted throughout the TT, which might be attributed to running engaging more muscle mass than cycling. Moreover, the gastrocnemius muscle isn’t depleted for glycogen, and fewer glycogen is broken down within the leg muscles.

Future studies are needed to account for various sexes and use different macronutrient intake periodizations to higher understand the mechanisms related to the advantages of those different diets. Metabolomics studies could also elucidate the continued adaptions in metabolism.